Microcomputer having a memory bank switching apparatus for accessing a

Abstract

A memory expansion scheme is provided which permits a program to automatically cross memory bank boundaries, without user intervention. A memory bank address register stores a value corresponding to a selected memory bank (i.e. Bank 0), in a 4-bit subfield (K-Field). In the preferred embodiment, the K-Field is implemented using six (6) bank number registers, each of which is coupled to the corresponding address register, to form a 20-bit (extended) logical address. During an effective address calculation, in the index addressing mode, a 16-bit logical offset address, stored in an offset register, is added to the 20-bit (extended) logical address, by an adder in the ALU. The adder transfers a 20-bit physical address onto an address bus, via an address buffer. When the calculated address crosses a memory bank boundary, the upper four (4) address bits (A.sub.16 -A.sub.19) are automatically updated, thereby enabling the program to cross a memory bank boundary without user intervention. Furthermore, in selected cases, the contents of the bank number register (address bits A.sub.16 -A.sub.19) are automatically updated. Thus, the memory bank expansion scheme provides the capability to automatically cross memory bank boundaries, while automatically updating the information stored in the bank number register.